The cytoplasmic ground substance has been examined in whole cell preparations by high voltage electron microscopy and has been found to comprise a three-dimensional lattice of slender (30-60 A) trabeculae. This network divides the ground substance into two phases, a protein-rich, polymerized phase comprising the microtrabecular lattice (MTL), and a water-rich fluid phase which fills the intertrabecular spaces. The trabeculae of the lattice are then coextensive with the subplasmalemmal cortex of the cytoplast and with the surfaces of the ER cisternae and microtubules. It contains and supports within its meshes these formed structures and the free polysomes. It is the general purpose of this study to achieve a better understanding of this system of the cell and most especially to examine: 1. the response of the observed microtrabecular lattice to Ca ions and Mg ions; b. ATP requirements for the maintenance of the normal form of the lattice; c. the involvement of the lattice in the translocation of the granules in polymorphonuclear leukocytes (PMN's) and the distribution by immunocytochemical procedures, of cAMP and cGMP-dependent protein kinases relative to the MTL during phagocytosis and granule motion, and d. the relation of the lattice to the differentiation and assembly of myofibrils and the distribution within the MTL of polysomes engaged in actin and myosin synthesis.